Micro-Injection Molding Puts Ceramics in Top Form

Ceramic injection molding permits the economical production of ultra-precise small components while affording maximum design freedom.

Figure 1. Complex, as-sintered orifice geometries are possible.

In recent years, fine industrial ceramics have been established as a forward-looking material in almost every industry. Ceramic injection molding (CIM) offers an outstanding opportunity to make the optimum use of the advantages provided by these materials.

In order to fully exploit the potential offered by this modern ceramic forming process, however, comprehensive know-how is needed throughout the entire processing sequence. It is essential to understand the production of the raw material, the design and manufacture of the molding dies, the CIM process itself, and the finishing of the component and its quality control.

Applications

In general, CIM provides a distinctly economical and reliable production technique, particularly for medium-sized unit quantities (and larger) that involve complex shapes, stringent tolerance standards, thin walls and tiny holes (see Figure 1). The use of ceramics in existing or new products can be recommended to meet the following requirements:

Bio-compatibility

Corrosion resistance

Electrical insulation

Wear resistance

Quality of finish (e.g., polished, net shape)

Thermal stability

High modulus of elasticity with low weight.

In the medical and dental industries, particular value is placed on bio-compatibility, transparency, coloration, quality of finish, and consistency. These applications place exacting demands on the purity of the materials, process reliability and quality assurance. There is no shortage of applications for ceramics in these fields, due to the widespread need for such implementations as dental tooth implants, abutments, orthodontic brackets, medical endoscopic devices and nozzles for analytical requirements.

In the field of machine manufacture, characteristics such as extreme hardness and resistance to wear, corrosion and chemicals-together with exceptional strength and low weight relative to volume-count for a great deal. Examples in this category include nozzles, guides, gearwheels and parts with screw threads.

The electronics sector calls for effective electrical and thermal insulation, electrostatic discharge properties, and geometrically precise dimensions for particularly small components. Applications can be found in bonding capillaries, receptacle guides and grippers, among others.

Figure 3. Nozzles for micro-dispensing incorporate complex features that were determined early in the design stage.

Material Characteristics

Apart from a variety of specific, application-related mixtures, two basic materials in particular have become established in the field of oxide ceramics. Aluminum oxide (Al2O3) is currently the most important oxide ceramic material. It stands out by virtue of its high strength and hardness, high wear resistance, corrosion resistance, high thermal conductivity, high temperature resistance, and outstanding electrical insulation properties. Similarly, zirconium oxide (ZrO2) is principally selected due to its exceptional bending strength, high modulus of elasticity (comparable with steel), low thermal conductivity, high temperature resistance, and good tribological characteristics.

Both of these materials provide the possibility of individually adapting and producing feedstocks for the injection process and adapting them to the mixture ratio, grain size and binding agent. This flexibility enables manufacturers to optimally match the properties of the material to the requirements of the finished part.

The CIM Process

The ceramic injection molding process offers a great deal of design freedom. With this process, arbitrary shapes such as internal and external threads, undercuts, inclined drill-holes and freely formed faces can be directly produced without incurring any reworking costs.

The oxide powder or powders are mixed in varying proportions with a binding agent. The resulting feedstock must be suitable for injection molding at a high green density, and the binding agent must be able to be removed before the sintering process takes place.

CIM is similar to plastic injection molding and is carried out on optimized microinjection molding machines. Achieving the optimum green density is essential. Usually, the binding agent content is carefully removed from the molded blank in accordance with a thermal/time curve. In this process, the binding agents are driven off through thermal decomposition or by means of a combination of extraction and pyrolysis. When freed of these agents, however, the component must retain precisely the shape it acquired in the injection molding process.

With the sintering process, compaction (without pressure) assumes almost the same theoretical density values as the pure material itself. At the same time, provision must be made for a linear contraction of 20-30%. The sintering of oxide ceramics is carried out in air or a vacuum; in this case, sintering contraction is dependent on the material and the green density achieved for the injection molding process.

With the aid of hot isostatic presses (HIPs), the microstructure can be retrospectively compacted again under heat and pressure (the last 0.5%) for special applications. After they have undergone thermal treatment, the component parts possess the properties of the pure material itself (e.g., hardness, density, compression resistance, solidity, resistance to chemicals, freedom from distortion, and thermal and electrical properties).

After the sintering process, CIM blanks essentially correspond to the finished part. If necessary, additional work may be carried out to remove any sprue or to produce a special product feature. All of the common processing methods can be used for finishing operations, such as grinding, lapping, honing and polishing.

Designing for CIM

In order to make the optimum use of CIM technology, the following points should be observed at the design stage for the component part:

Unnecessary and pronounced variations in wall thicknesses and abrupt changes of cross-sections should be avoided.

Accumulations of material should be circumvented (incorporate recesses).

Sharp edges should be rounded off wherever possible.

Lengthy, freestanding cores should be made as symmetrical as possible.

Figure 2 depicts alumina insulators for surgical devices that have retained their shape in the injection molding process without any additional finishing work. Figure 3 illustrates a component that has been designed for CIM in conjunction with the customer from an early stage.

Expanded Possibilities

The shaping possibilities offered by the CIM process are comparable with those of plastic or metal micro-injection molding. Because of their special properties, however, fine ceramics open up entirely new possibilities.

Developments in recent years indicate that increasing numbers of users from the most wide-ranging industries are making use of these possibilities for their products and are substituting fine ceramics for conventional materials. In doing so, manufacturers are pursuing the objective of endowing their products with better usage characteristics and, in turn, reinforcing their competitiveness.